The invention relates to an impeller that is generally used in reactors in hydrometallurgical processes.
The impeller of the invention is meant to be used for the same purpose as the straight-bladed impeller conventionally used in the field, and certain other energy-efficient impellers known in the field. The impeller of the invention can be used for example in large-scale applications, instead of the old conventional impellers.
The reactor is a mixing tank, in which the process solution is fed for the desired procedure. In addition to the impeller, the reactor generally includes flow tumblers attached to the walls.
The impeller is mainly used in arrangements where the process solution is solid substance, for example created as a result of precipitation. In this kind of operational environment, it is important that the mixing pattern is sufficiently strong throughout the whole reactor area, in order to agitate the solids so that they are not accumulated on the bottom, for instance.
In addition, it is necessary to create turbulence in the process, in order to make the reactions required by the process happen. In certain application environments, it is necessary to avoid excessive agitation power in the solids mixture, in order to prevent the breaking up of process elements, such as flocculants.
For example, the breaking up of solid particles created in precipitation, and of flocculants possibly used in the process, would strongly reduce the efficiency of the precipitation. When the precipitation efficiency is reduced, more and more flocculants must be added in the process in order to increase the efficiency, which further increases the expenses. The process solution is composed of for example an aqueous solution containing acid and substances dissolved therein. In addition, it is possible that gas is blown into the reactor, in case it is necessary for the reactions.
As possible process environments, there are suggested the precipitation of cobalt and nickel, production of lime milk, solution processes and large-scale sewage treatments. In a precipitation process, the metal is brought in the process solution for example by means of bulk leaching.
In a precipitation reactor, the metal is attempted to be precipitated in a process solution for example by blowing therein a gas, such as hydrogen sulphide. Now the metal contained in the solution begins to nucleate. When nuclei are created in the solution, they start to grow, and after surpassing a certain size, they can be separated from the process solution in thickening devices.
Generally known state of the art is represented by the patent publication U.S. Pat. No. 5,052,892 by Chemineer Inc. The described impeller element includes 2-4 blades, generally 3. The aim of the impeller is to minimize the power used for creating axial flow, so that the micromixing needed by the reactions need not be observed, and the aim is achieved by a smaller number of blades. The total edge of the impeller blades is small, i.e. 25-30 degrees, and the impeller blade blanks are rectangular in shape, i.e. the front and rear edges of the blades are in parallel.
According to the reference publication, the axial efficiency of the impeller is based on radial concavity, which is achieved by means of a diagonal fold. According to said reference publication, the blades are not attached to the impeller axis, but the blades are fastened by a bolt joint to a hub surrounding the axis, in which case the hub must be provided with protrusions to which the blades are attached to.
In an impeller according to the publication U.S. Pat. No. 5,052,892, the blade is provided with a bend line that is made in parallel with the blade edges, which bend line extends from the blade root to the tip and divides the blade in a front portion and a rear portion. The front portion of the blade is further provided with another bend line that extends diagonally from the tip of the first bend line to the front edge of the blade and ends at a distance that is roughly three quarters of the blade length, so that the end point of the bend line is at the distance of ¼ of the blade length from the hub. The angle of the first bend line is of the order 10-25°, and the angle of the second bend line is of the order 5-15°. In material, the blades are not uniformly thick throughout, but both their front and rear edges are chamfered in order to reduce the resistance. In the front edge of the blade, the chamfering is made on the upward side, and in the rear edge on the side underneath.
One of the drawbacks of the impeller described in the publication U.S. Pat. No. 5,052,892 is that it creates a powerful energy peak at the tip of the blades, i.e. agitation at the blade tips is very powerful. Thus the mixing effect is not distributed evenly in the reactor surrounding the impeller. The structure of the blades provided with chamfering requires several successive production steps.